> How can I set up this mouse here, on the NetBSD system? Your kernel has presumably / hopefully probed your ps/2 connected mouse already. You can probably find an instance of "pms" in /var/run/dmesg.boot.
On NetBSD, we have an "overlay" which covers both PS/2 and USB mice (among others), namely wsmouse. See e.g. the pms(4) and wsmouse(4) man pages. This means that e.g. the X server only needs to relate to wsmouse, and not know exactly what underlying driver is used to drive the mouse. Regards, - Håvard